The invention relates to a turbine blade with a corrosion resistant protective coating of MCrAlY.
During the operation of high temperature gas turbines, the turbine blade surfaces reach temperatures of up to 900.degree. C. At such high temperatures, the principal corrosion mechanism resides in oxidation (diffusion of oxygen). The blades are therefore coated by a high temperature super alloy MCrAlY(M=metal basis, for example Ni, Co).
Protective MCrAlY coatings are generally applied by a plasma spray coating process. The alloy solidifies in a two phase form. This provides for a good basis for the formation of AL.sub.2 O.sub.3 cover layers on the surface. On the surface of the two-phase alloy, the formation of a homogeneous oxide layer is inhibited. The oxide cover layers, which are formed on the surface, are subject to spalling.
R. Sivakumor, Princ. of Scientific and Mat. Process, Vol. 2, Page. 671-726 discloses that this two phase layer can be converted to a single phase layer in a melt conversion process using laser beams. The disadvantage of this method is that the laser beam covers only a small area (at the power densities required herefor, which is 10.sup.5 -10.sup.6 W/cm.sup.2) of &lt;10.sup.-2 cm.sup.2 and the low penetration depth of the laser beam into the material.
The small area energy input results in excessive thermal tensions which lead to the formation of cracks in longitudinal as well as transverse directions. Cracks reduce the spallation resistance of the oxidation layers and, consequently, of the corrosion resistance.
Also, the small laser beam diameter results in the formation of beads on the surface, in phase separations, and recrystallizations because of the raster movement of the laser beam on the surface.
The relatively long irradiation time of several milliseconds for the melting of a layer of 10 .mu.m thickness results in a change of the original stoichiometry in the layer, that is it leads to a reduction of the content of the light elements (Al,Y) which, by convection, are floated to the surface and consequently, are not available for the renewal of the oxidation layer.
It is the object of the present application to provide a turbine blade with a coating which is not subject to spalling.